BIALLELIC EXPRESSION OF IMPRINTED GENES IN THE MOUSE GERM-LINE - IMPLICATIONS FOR ERASURE, ESTABLISHMENT, AND MECHANISMS OF GENOMIC IMPRINTING

Genomic imprinting in mammals determines parental-specific (monoalleli c) expression of a relatively small number of genes during development . Imprinting must logically be imparted in the germ line, where inheri ted maternal and paternal imprinting is erased and new imprinting esta blished according to the individual's sex. We have assessed the allele -specific expression of four imprinted genes, two of which exhibit mat ernal-specific (H19 and Ig/2r) and two of which exhibit paternal-speci fic (Ig/2 and Snrpn) monoallelic somatic expression, in the germ line of F-1 hybrid mice utilizing quantitative RT-PCR single-nucleotide pri mer extension assays. The expression of each gene was biallelic in the female and male germ line from the time that migratory mitotic PGCs e ntered the embryonic genital ridge and throughout gametogenesis, excep t that H19 RNA was not detected late in gametogenesis. These findings demonstrate that inherited imprinting is erased, or not recognized, in germ cells by the time of genital ridge colonization; also that new i mprinting may not be established until late in gametogenesis, or that it is incomplete or not recognized at this stage. Regardless of imprin ting status, a generalized neutralization of imprinting is evident in the germ line, associated with the totipotent state of this unique cel l lineage.